A capacitor-charging circuit utilizing non-linear power supplies often requires a current-limiting resistor which can not be estimated by the solution of simple linear-circuit equations. Consequently, a process is required to determine the appropriate value of the current-limiting resistor which is both sufficient to limit the current to protect all heat-sensitive or current-sensitive components, and also not too stringent so as to maximize power transfer efficiency, thus also lowering cooling requirements for the final circuit.
Power supplies for charging capacitors generally have a means of limiting periodic inrush current to prevent component failures. While there are many options for this, such as inductors, transistors, or varying supply voltages, the cheapest is often a resistor. The selection of the resistor for a linear power supply is simple: one solves the linear circuit equations for the greatest efficiency, or chooses the value providing sufficient current limiting to protect all components, or a cost-efficient combination of these constraints. However, if the supply is non-linear, such as those designed for constant-current, high-voltage outputs, in general the problem can not be solved unless all of the supply circuits' parameters are known. These are often not known due to normal practice of keeping the internal circuits a competitive secret.